Recent advances have been made in the field of producing three-dimensional objects, such as prototype parts and finished parts in small quantities, directly from computer-aided-design (CAD) data bases. Various technologies are known to produce such parts, particularly through the use of additive processes, as opposed to subtractive processes such as conventional machining. An important additive process for the production of such objects is selective laser sintering, recently commercialized by DTM Corporation. According to the selective laser sintering process, a powder is scanned in layerwise fashion by a directed energy beam, such as a laser, to fuse the powder at selected locations corresponding to cross-sections of the object. The scan of the laser across the target surface is generally in raster scan fashion, with the laser beam modulated on and off at locations corresponding to the cross-section of the object in that layer; alternatively, the laser may be operated in a vector mode so as to "draw" the object cross-section in the powder layer. In either case, fused locations within each layer adhere to fused portions of previously fused layers, so that a series of layers processed in this manner results in a finished part. Computer control of the scanning of the energy beam thus enables direct transfer of a design in a computer-aided-design (CAD) data base into a physical object.
The selective laser sintering technology is described in further detail in U.S. Pat. No. 4,247,508 issued Jan. 27, 1981, now assigned to DTM Corporation and incorporated herein by reference, and in U.S. Pat. Nos. 4,863,538 issued Sep. 9, 1989, 5,017,753 issued May 21, 1991, 4,938,816 issued Jul. 3, 1990, and 4,944,817 issued Jul. 31, 1990, all assigned to Board of Regents, The University of Texas System and also incorporated herein by this reference. As described in the above-noted patents, and also in U.S. Pat. Nos. 5,156,697 issued Oct. 20, 1992, 5,147,587 issued Sep. 15, 1992, and 5,182,170 issued Jan. 26, 1993, all also assigned to Board of Regents, The University of Texas System and incorporated herein by this reference, various materials and combinations of materials can be processed according to this method, such materials including plastics, waxes, metals, ceramics, and the like. In addition, as described in these patents and applications, the parts produced by selective laser sintering may have shapes and features which are sufficiently complex as to not be capable of fabrication by conventional subtractive processes such as machining. This complexity is enabled by the natural support of overhanging fused portions of the object that is provided by unfused powder remaining in prior layers.
Further refinements in the selective laser sintering process, and advanced systems and machines for performing selective laser sintering, are described in U.S. Pat. Nos. 5,155,321 issued Oct. 13, 1992, commonly assigned herewith, 5,155,324 issued Oct. 13, 1992, and International Publication WO 92/08592, all of which are incorporated herein by reference. Copending application Ser. No. 789,358, filed Nov. 8, 1991, now U.S. Pat. No. 5,252,264 commonly assigned herewith and incorporated herein by this reference, further describes an advanced apparatus for selective laser sintering in which powder is dispensed from either side of the target surface.
Another laser-based process for forming of three-dimensional objects is commonly referred to as stereolithography. According to the stereolithography technique, as described in U.S. Pat. Nos. 4,575,330 and 4,929,402, a directed light beam, such as a laser operating at ultraviolet wavelengths, is used to cure selected portions of the surface of a vat of photopolymer.
Success in the practice of laser-based processes, including both the selective laser sintering technology and also stereolithography, depends upon the faithfulness with which the object is produced relative to the CAD representation. Accordingly, investigation of the sources of error, and the ability to correct for such error, has become important, especially for high resolution and finely detailed objects.
Several significant types of error in the fabrication of objects by selective laser sintering have been discovered. A first type of error is geometry-dependent error, which is based upon the angle from the vertical of the laser beam. Fundamental plane geometry indicates that the linear displacement of the irradiated location of the target surface is nonlinearly related to the angle of the laser beam from the vertical, and thus is nonlinearly related to angular displacement of the beam. As is well known in the art, conventional scanning of a laser beam across a plane, using a pair of planar mirrors oriented to deflect the beam in two dimensions, will draw an arc with a radius dependent upon the distances of the mirrors from the target plane. The projection of this arc onto the image plane results in a line having a distance nonlinearly related to the angle swept by the mirror. Without compensation of this nonlinear relationship, a segment drawn by the laser on the target surface at an angle from the vertical will not correspond to that in the CAD data base. For example, a square drawn around the perimeter of the plane will be distorted according to the well-known "pincushion" effect.
A second type of imprecision results from a mismatch between the resolution of the image field and that of the laser scanning system, in the case where scanning correction is performed as a function of the scanning field. If the number of correction points are defined according to the digital resolution of the scanning system, where the image field is somewhat smaller than that scannable by the scanning system, the number of possible correction locations is reduced from its optimal density.
A third type of error is scanner error that is a function of distance on the image plane. For example, scanner errors such as linearity error and gain scaling error will increase with the length of the vector or line drawn on the image plane. Distance-dependent error of this type will result in vectors drawn on the image line which have dimensional errors that are a function of (e.g., a percentage of) their length, as opposed to an absolute error value. These distance-dependent errors and their variation over the image field strongly depend upon the dimensions of each specific scanner, and do not behave according to a theoretical function. As such, accurate correction for distance-dependent error requires empirical characterization of each system.
A fourth error type corresponds to time-dependent scanner error and temperature-dependent error. Errors such as gain drift and offset drift are examples of this fourth error type. Physical causes of such drift include slight mirror rotation or optic movements due to vibration, thermal expansion, or bumps to the system, resulting in scanner positioning error. These errors tend to be dependent upon the environment in which the machine is installed, and changes in this environment over time.
These types of error are also present in the stereolithography process. By way of further background, various techniques have been used in the field of stereolithography to calibrate and normalize a stereolithographic apparatus. U.S. Pat. Nos. 5,058,988, 5,059,021, 5,123,734, and 5,133,987 describe such calibration and normalization techniques. One such technique utilizes pinhole beam profile sensors mounted outside of the resin vat, at which the laser beam is periodically directed and a correction factor calculated. Another technique disclosed in these references is the use of a plate placed at the target surface (instead of the resin), where the plate has pinholes therethrough, behind which photodiodes are located to measure and communicate the laser beam intensity at target surface locations, thus allowing the calculation of correction factors.
Such prior techniques insert interpolation errors into the derivation of the correction factors, however, to the extent that they rely upon beam profile sensors located outside of the target surface (e.g., mounted outside of the resin vat), particularly since error is generally a continuously varying function over both axes of the image field. As such, it is greatly desired to take direct measurement of the beam location over the target surface. However, prior techniques such as those discussed above relative to the use of a plate with pinholes in the stereolithography technology require cumbersome and computationally intensive procedures, and may require significant setup of the machine (e.g., removal of the resin vat) to perform.
In addition, it is believed that the use of the pinhole plate apparatus is inherently limited in its accuracy. This is because the energy level of a laser beam over its width tends to be quite uniform within its spot, thus making it difficult to distinguish the true center of the laser beam spot relative to locations within the laser beam spot that is some distance away. Considering that the energy level inside of the laser beam is extremely high relative to the energy outside of the beam, the necessary dynamic range of conventional sensors will limit the ability to discern two relatively high energy levels from one another. Furthermore, it has been observed that variations in laser power over time are greater than the difference between the energy at the true center of the spot and the energy within the spot but away from the center. As such, it is contemplated that this conventional approach is quite limited in accurately correcting for scanner error of the above-described types. Furthermore, it is contemplated that the use of this conventional technique for lasers of longer wavelength (e.g., CO.sub.2 lasers) will be prohibitively expensive, considering the cost of the detectors for such wavelengths. In addition, the larger spot size of such longer wavelength CO.sub.2 lasers adds further complications to this conventional technique.
It is therefore an object of the present invention to provide an automated method for calibration and normalization of scanner errors in a laser-based object fabrication apparatus.
It is a further object of the present invention to provide such a method which accounts for multiple sources of significant scanning error.
It is a further object of the present invention to provide such a method which may be used without requiring significant setup effort.
It is a further object of the present invention to correct for error of the geometry-dependent, distance-dependent, and time-dependent type, while maximizing the correction cell density.
It is a further object of the present invention to provide such an automated method suitable for time-dependent monitoring of error conditions.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.